1. Field of the Invention
The present invention relates to a differential gear permitting differences in speed of rotations for example between the right and left or the front and rear drive wheels of automobiles, and to a method of manufacturing disc-like members for use in the differential gear.
2. Description of the Related Art
Typical automobile differential gears have a pinion gear interposed between a pair of bevel gears coupled to the output shaft so that when the pinion gear shaft is subjected to an external rotational force, the pinion gear is rotated upon the differential motions to permit the difference in speed of rotation between the output shafts. Known as a differential gear having a differential limiting function for restricting the idling of one of the drive wheels during turning or at the travel on the surface of road having a lower frictional force is one having an additional differential limiting mechanism such as for example a viscos coupling making use of resistance of the viscos fluid. However, the structure having such an additional specialized mechanism as the differential motion limiting mechanism often suffers from drawbacks of increases in size and cost of the gear body. Thus, the applicant has already proposed a differential gear smaller in size than the conventional structure and capable of achieving the differential limiting effect without needing any additional specialized mechanism (Japanese Patent Laid-open Pub. No. Hei8-170705).
This differential gear comprises a gear case which is rotated by an external force; a pair of disc plates confronting each other and coaxial with the gear case; a plurality of balls arranged between the confronting faces of the pair of disc plates; and a center plate for retaining the balls and rotating together with the gear case. The center plate includes a plurality of elongated holes extending in the radial direction of the disc plates and movably accommodating the balls. The confronting faces of the disc plates are provided with grooves with which the balls are rollingly engaged. The rotational force is transmitted by way of the balls and the grooves to the disc plates. The balls are caused to roll along the grooves of the disc plates while reciprocating within the elongated holes of the center plates, to thereby permit the difference between speeds of rotation of the two disc plates. The differential motion between the disc plates is limited by the axial reaction force generated between the balls and the grooves. Accordingly, this differential gear is advantageous in reduction in number of constituent parts and compactness as well as easy assembling since it effects the differential motion between the disc plates by the engagement of the balls with the grooves and use is made of the mechanical characteristics in that configuration to obtain the differential limiting effect.
In the automobile drive wheels, for example, the differential limiting forces in the differential gear occur both during the acceleration and deceleration. When the directions of transmission of rotational forces to the drive wheels differ between during the acceleration and deceleration due to the automobile travel performances, objects of use, or combinations with the attached mechanisms, it would be desirable that the magnitudes of the differential limiting forces also be set so as to differ from each other. In the automobiles equipped with an ABS (anti-lock braking system) for preventing any slip during braking operation, it would be necessary to prevent the drive wheels from being locked by reducing the differential limiting force which may occur during the deceleration in view of controlling the braking force relative to the surface of road on a wheel-to-wheel basis.